Data on lightning and thunderstorm electrical activity, such as occurrence frequency, spatial distribution and temporal properties, have become available in significant quantity and detail in recent years. Reference is made, for example, to Mach, et al. "Site Errors and Detection Efficiency in a Magnetic Direction-finder Network for Locating Lightning Strikes to Ground," J. Atmos. Oceanic Technol. 3, 67-74 (1986); and Orville, et al. "Cloud-to-Ground Lightning Flash Characteristics from June 1984 through May 1985," J. Geophys. Res. 92, 5640-45644 (1987). More recently, it has been recognized that polarity patterns of lightning occurrence in mesoscale systems appear to be related to the direction of the upper level winds. Reference is made to Orville, et al. "Bipole Patterns Revealed by Lightning Locations in Mesoscale Storm Systems," Geophys. Res. Lett., (Feb., 1988).
In view of this recent accumulation of data, there is a growing awareness that return stroke polarity may be an important indicator of certain phenomena associated with thunderstorms. For example, there is evidence that the polarity of lightning strokes, i.e., the return stroke polarity of lightning, may be an indicator of the progress of a thunderstorm and, hence, an understanding of conditions which lead to the dominance of one polarity or the other is of importance. One result of recent studies involving return stroke polarities is the finding that positive ground stroke currents constitute approximately 1 percent of the total return strokes in summer storms, but dominate in winter storms, with current rates being as high as 95 percent.
Previous studies to determine polarity of return strokes of lightning have been limited to 600 km or less because of the attenuation of the lightning ground wave over distance. Errors in determining polarity of distant lightning (occurring at greater than 600 km away) can occur because the ionospheric reflection of the lightning stroke, unlike the ground wave, is not attenuated much but is usually reversed in polarity.
Studies of distant lightning (occurring at greater than 600 km away) is generally performed by measurement of the associated electric fields in the form of radiated waveforms. Return strokes of both negative and positive polarity exist in such distant radiated waveforms, and an effort has been made to provide closer examination of the waveforms of the radiation fields. One observation that has been made based on this examination is that many lightning VLF waveforms from a distance beyond 500-600 km exhibit well-defined, positive or negative slow tails, with a slow tail being defined as a low frequency dispersion effect occurring in the zeroth mode of the earth-ionosphere wave guide.
While the observation of the slow tails in a lightning electric field dates back to 1926, numerous questions regarding some of their properties have remained to be explained, including the important question of determining the specific source of this radiation.